1. Field of the Invention
The invention relates to flexible rotary shafts. More particularly, the invention relates to a multifilar flexible rotary shaft having reduced hysteresis, increased torque transmission, and low internal friction. The shaft of the invention is particularly useful as a component of minimally invasive surgical instruments which must traverse a tortuous path.
2. State of the Art
Flexible rotary shafts are used in many applications in order to transmit a torque through a curved path. Generally, a flexible rotary shaft has an input end which is coupled to a source of rotational energy (e.g. a motor) and an output end which is coupled to something to be rotated. In some applications, a single, monofilar, wire is used. A monofilar flexible rotary shaft must have sufficient yield strength to resist permanent distortion when bent around a specified radius of curvature, or “radius of operation”. Indeed, when designing a monofilar flexible rotary shaft, the designer must first determine the radius of operation, i.e. the smallest radius the shaft will be expected to traverse. The maximum wire diameter for the radius of operation can be determined solely on the yield strength and modulus of elasticity of the wire used. A wire of a given material having a diameter larger than this maximum would be permanently deformed if bent around the radius of operation.
Once the maximum diameter of a monofilar flexible rotary shaft is determined for a particular radius of operation, the designer must determine whether the wire has strength and torsional stiffness for a particular application. Monofilar flexible rotary shafts are notoriously inadequate for transmitting a relatively large torque through relatively small radius of operation.
Traditionally, multifilar flexible rotary shafts have been employed for transmitting a relatively large torque through relatively small radius of operation. Prior Art FIGS. 1 and 2 illustrate a simple multifilar shaft. A typical multifilar flexible shaft 10 consists of a plurality of wire filaments, e.g. 11, 12, 14 16, 18, 20, wound, typically about a center filament, e.g. 22, in a helical organization. Though not shown in prior art FIGS. 1 and 2, often several layers of filaments are wound in alternating opposite directions. While these constructions overcome the deficiency of monofilar flexible rotary shafts, they nevertheless have their own disadvantages. The most notable disadvantage of multifilar flexible rotary shafts is increased hysteresis which results from internal friction among the filaments. Hysteresis is the term generally used to describe the difference in the behavior of the input and output ends of a flexible rotary shaft. In its simplest form, hysteresis refers to a time delay between the application of torque at the input end and the resulting rotation of the output end. Hysteresis also refers to other, erratic, behavior of the output end which is not identical to the behavior at the input end.
Internal friction and hysteresis in multifilar flexible rotary shafts results from the manner in which they are constructed. Specifically, the individual wires are deformed during the winding process so that they bear against one another in such a way that the assembly “holds itself together.” That is, if one disassembles such a flexible shaft, one will find that the individual wires are deformed into a helical shape, and each layer grips the next inner layer with a certain amount of compression. This type of construction is known as a “pre-formed” cable, because the individual wires are formed into helical shapes during the stranding process. If such cables were not made in this way, they would be very difficult to handle as a subassembly because they would spring apart when the ends were cut. In fact, some flexible shafts do spring apart to some extent when cut, but in all known multifilar flexible shafts the individual wires are permanently deformed into a helical shape. Because of this, there are considerable compressive contact forces among the wires, resulting in friction between the wires when the shaft is rotated while traversing a curved path. This internal wire-to-wire friction results in energy absorption in the flexible shaft, so that energy delivered to the output end is less than the energy applied to the input end.
It is known that the filaments of a flexible shaft generate internal friction which increases as its radius of operation decreases. Further, for a given radius of operation, the more flexible the shaft, the lower will be the amount of internal friction. The torque needed to overcome the resistance due to this internal friction is called the “torque to rotate.” As such, the torque-to-rotate value of a given shaft is normally specified for a specific radius of operation. It follows, therefore, that the more flexible the shaft, i.e., the higher the bending flexibility, the lower will be the torque to rotate for a given radius of operation.
Torsional stiffness and torsional deflection denote inverse parameters of a flexible shaft. Torsional stiffness specifies a measure of the resistance of the shaft to an applied torque, i.e., a twisting or a torsional force, about its rotational axis. Torsional deflection designates the degree of twist per unit length that a flexible shaft will experience due to an applied torque. The torsional deflection is usually expressed in degrees per foot per pound-inch (deg/ft/lb-in); its inverse, torsional stiffness, is expressed in units of lb-in/ft/deg.
Therefore, when choosing a flexible shaft, the length of the smallest radius of operation and the magnitude of the input torque are important factors in determining the bending flexibility of the shaft. The following conditions should be met when selecting a flexible shaft: first, the shaft must have sufficient bending flexibility so as not to be damaged when flexed into its smallest radius of operation; second, the shaft must have sufficient flexibility so that the torque-to-rotate value at the smallest radius of operation is at least less than the input torque, i.e., the output torque of the driver element; and third, the shaft must have sufficient torsional stiffness to accurately transmit rotary motion with a minimum of torsional deflection.
Most designers believe that the mechanism of torque transmission along a multifilar flexible shaft is by means of tensile (and compressive) stresses in the individual wires. In fact, for existing multifilar flexible shafts transmitting large torques this is essentially true. When subjected to high torques, such multifilar assemblies react by some layers expanding and some contracting (depending upon the direction of twist). If the inner are expanding layers, they are resisted by outer contracting layers, so the torque is resolved into tensile stresses in the contracting layers and compressive stresses in the expanding layers. This reaction to torque thus results in contact forces between the layers of wire, and these contact forces result in friction. As a result, there is noticeable lost rotary motion at the output end of the flexible shaft when the input torque is alternated from one direction to the other, as in the case of a shaft used to steer a medical device or to transmit a rotary position signal. When the input of the shaft is twisted in one direction from its static condition, a certain amount of twist (hysteresis) is required to overcome the internal friction and cause the layers to come into interference with one another before the rotary motion is observed at the output end. If the shaft is then twisted in the alternate direction, the previous hysteresis must first be overcome to return the internal state of the wires to its static condition; then, a similar amount of hysteresis is introduced as the shaft is “wound up” in the new direction. This hysteresis is made worse when the flexible shaft traverses a curved path, because additional stresses between the layers (resulting in increased internal friction) are introduced by the bending of the shaft through a curved path.
The previously described hysteresis in a pre-formed multifilar flexible shaft prevents it from working as a precise transmitter of rotary motion from one end to the other. While these shafts may work well enough in the unidirectional transmission of power, they are ineffective in precisely transmitting rotary control (in two rotational directions) because of the hysteresis or “lost motion” caused by their internal friction.